The present invention is particularly useful within the environment of a digital tape recording system employing a single-reel tape cartridge. In one pertinent form, a magnetic tape drive storage subsystem for handling a single-reel tape cartridge is shown in FIG. 1. The FIG. 1 subsystem is generally comprised of a rectangular housing that has a common base carrying two spindle motors. A first spindle motor rotates a permanently mounted take-up reel dimensioned to accept a relatively high speed streaming magnetic tape. A second spindle is adapted to rotate a feed reel of the single-reel removable tape cartridge. The removable tape cartridge is manually or automatically inserted into the FIG. 1 tape drive via a suitably-dimensioned slot formed in the drive's housing. Upon insertion of the tape cartridge into the slot, the cartridge tape feed reel becomes engaged by the second spindle motor. However, prior to synchronized rotation of the first and second spindle motors, the tape cartridge leader becomes automatically buckled to a drive leader extending from the take-up reel along the tape path within the drive. A mechanical buckling mechanism, as shown and described in commonly owned, copending U.S. patent application Ser. No. 08/666,854, entitled, "Improved Tape Buckling Mechanism For Single Reel Cartridge Tape Recording" is presently preferred as the buckling mechanism for coupling and uncoupling the tape drive and cartridge leaders. The disclosure of U.S. patent application Ser. No. 08/666,854 is hereby incorporated herein by reference.
A number of rollers are positioned along the tape path between the tape cartridge and the take-up reel guide the magnetic tape as it traverses at relatively high speeds (e.g. 100 inches per second or faster) back and forth between the tape cartridge and the permanently mounted spool. The tape drive further comprises a read/write head mounted on a tape head carriage-actuator assembly which is also located between the take-up reel and the tape cartridge tape feed reel. As shown for example in commonly assigned U.S. Pat. No. 5,371,638, the tape head is positioned laterally and rotationally relative to the tape, thereby enabling recording and playback on a multiplicity of parallel longitudinal data tracks in either a non-azimuthal or an azimuthal recording pattern. The disclosure of U.S. Pat. No. 5,371,638 is incorporated herein by reference. A bidimensionally operable tape head carriage is described in commonly assigned, copending U.S. patent application Ser. No. 08/557,662 filed on Nov. 13, 1995, for "Tape Drive Head Positioning Device for Adjusting Head Tilt and Azimuth", the disclosure thereof being incorporated herein by reference.
During operation, the magnetic tape streams back and forth at high speed between the take-up spool and the tape cartridge along the defined tape path. Further, as the tape streams back and forth, the read/write head stores or retrieves data to/from the magnetic tape. One beguiling difficulty of high speed lineal streaming tape is that of lateral tape motions or displacements at the head as the tape streams by in forward or reverse direction during operation of the tape drive. As the number of lineal tracks is increased without any corresponding increase in tape width, in order to gain increased data storage density, lateral tape motion becomes very problematic. One way to compensate for lateral tape motion or "LTM" is to provide a sequence of head position calibration routines once a cartridge is loaded into the tape drive.
During initialization of the tape drive following loading of a tape cartridge, a number of diagnostic processes are executed. One important and conventionally time-consuming routine is finding or establishing calibration tracks located in a header area before the beginning-of-tape (BOT) hole or in a trailer region after the end-of-tape (EOT) hole. Magnetic tapes generally comprise at least two calibration tracks, one for calibrating head initial position in a forward tape direction, and one for calibrating head initial position in a reverse tape direction. Specifically, the read/write head must first find a tape edge and then traverse the tape to determine if the tape has previously written calibration tracks. If the calibration tracks have previously been written, then the head servo adjusts head position to follow these tracks, so that the head will be or become aligned with an initial data track following the BOT hole. However, if calibration tracks are not detected by the read/write head, then a more involved and time consuming routine is executed by which the locations of the calibration tracks are determined and written with minimal positional error. This process of determining, writing and checking forward direction and reverse direction calibration tracks can take as much as 20 to 30 seconds when the cartridge is first loaded into the tape drive. Once written the calibration tracks essentially provide a registration point for all subsequently written data tracks and thereby provide compatibility between different tape drives and cartridges of the same product family or standard. An incorrectly positioned calibration track will cause the read/write head to misregister its relative position on the tape, thereby causing all subsequently written data tracks to reflect a like misregistration, with potential subsequent data losses.
The relative tape calibration track location is generally determined by first detecting the a top or bottom edge of the tape, which can be achieved within a specified margin of error of approximately 5-mils. Next, the head actuator assembly will step the read/write head a predetermined number of steps to arrive at the nominal calibration track position. If calibration tracks are magnetically recorded at about the nominal position, those tracks are read and provide head position error values to the head position servo. The head position servo then actuates e.g. a stepper motor which rotates a lead screw for precisely moving the tape head carriage laterally with respect to the tape. This process is iterative and continues until the tape head aligns with the previously written calibration track. Since in some tape drives a lateral offset occurs when tape travel direction is reversed, a reverse direction calibration track is referenced and followed to provide a reverse-direction head position calibration.
As noted above, recent advancements in magnetic tape drive technology have been facilitated by read/write head structures and data processing electronics which are capable of storing and processing increased lineal track and bit densities on magnetic tapes. If the magnetic tape head is employed to write the precise calibration tracks magnetically, a considerable overhead time may be required. While this prior approach is satisfactory, it is slow, and may slow down or delay data block transfers. Thus, a hitherto unsolved need has remained for a method for providing rapid head position calibration in a manner not requiring the tap drive head to write calibration tracks, and in a manner enabling backward compatibility with the prior approaches.